Because it prefers to dine on some of our valued crop plants, the pea aphid (Acyrthosiphon pisum) is considered a major pest—thus a Bad Guy from our perspective. Pea aphids are not without their enemies. Enemy number one is a parasitoid wasp, Aphidius ervi. As parasitoid wasps are wont to do, females provide for their offspring by depositing their eggs in the haemocoel of another insect, in this case a pea aphid. The wasp larva feeds and develops there for about a week, eventually killing the host. When the host viscera have been consumed, the larva causes the aphid’s cuticle to harden and dry. Soon, an adult wasp emerges from the aphid "mummy." Agriculturally speaking, parasitoid wasps are Good Guys, important partners in our integrated pest management strategies. You can buy wasp eggs for your fields by the hundreds or thousands online.

Early on it was noticed that aphid (A. pisum) clones vary greatly in their resistance to the wasps. In some, the development of the wasp larva is arrested and the host survives. Because the pea aphid was known to harbor Buchnera aphidicola as a primary endosymbiont and also as many as five other bacteria as secondary endosymbionts, it was suspected that this variation in resistance might be due to which endosymbionts are on board. (What makes a symbiont a primary rather than a secondary symbiont? That depends on who you ask. Some distinguish based on the evolutionary age of the symbiosis, others mention the obligate nature of the relationship for both partners.)